© 2015 International Rectifier
pins by a 1mA current source pulling down on VS
so that a supply voltage is produced across the
external capacitor CHVS to supply the high side
circuitry. A value of 22nF is recommended for
CHVS. The internal bias supply also dissipates
some power, which can be calculated from the
formula:
mAVP
AVDCBUSBIAS
1)10(
__
⋅−=
In order for the high side current sense circuitry to
function, a minimum bus voltage of 30V is
required to provide adequate bias supply current
and standoff voltage.
Thermal Considerations
Since the IRS2980S dissipates some power
during normal operation, temperature rise of the
IC die must be considered as part of the design
process.
The SO8 IC package has a maximum power
rating (PD) of 625mW, therefore the sum of P
REG
and P
BIAS
should not exceed this value.
The junction temperature should remain below
125°C to ensure operation within specifications.
The junction temperature is normally 10°C above
the case temperature for an SO8 package
therefore the case temperature should not exceed
115°C at maximum ambient.
The junction to ambient thermal resistance of the
package (RθJA) is 128°C/W. This would give an
80°C temperature rise without any thermal relief at
the maximum PD value of 625mW.
In order to reduce the junction temperature rise
thermal relief should be added around the
IRS2980S on the PCB. With adequate thermal
relief the die temperature rise can be greatly
reduced. The recommended method is to place
an area of copper on the opposite side of the PCB
to the IC in the same position with several vias
added underneath the IC to conduct heat through
to the other side.
Figure 4: Thermal relief example
In addition if the IRS2980S is used in an
application where the circuit is encapsulated in
thermally conductive filler the die temperature rise
is also greatly reduced.
Figure 4 shows an example of thermal relief
placed around the IRS2980S in a high voltage
application. The top side copper layer is shown as
red and the bottom side is blue.
With thermal relief ΔT becomes:
+
⋅
⋅=D
HSRJAR
HSRJAR
PDT
θθ
θθ
Where RθHS is the thermal resistance of the
thermal relief area or heat sink, which will
normally be lower than RθJA.
Dimming
The IRS2980S includes a PWM dimming
oscillator that generates a linear ramp waveform
at the RAMP pin with the frequency determined by
an external capacitor to COM (CRAMP). A DC
voltage is applied to the ADIM pin which is
compared to this ramp to produce a gating signal
that enables and disables the high frequency
switching of the MOSFET gate drive. By varying
the ADIM voltage the duty cycle is adjusted
allowing brightness to be adjusted from zero to
100%. This is accomplished by operating the LED
driver in burst mode and varying the duty cycle of
the bursts. The LED current during dimming is
shown in figure 5:
